Variable valve timing for egr control

ABSTRACT

Methods and systems are provided for adjusting cylinder valve timings to enable a group of cylinders to operate and combust while another group of cylinders on a second are selectively deactivated. Valve timing may be adjusted to allow flow of air through the inactive cylinders to be reduced, lowering catalyst regeneration requirements upon reactivation. The valve timing may alternatively be adjusted to enable exhaust gas to be recirculated to the active cylinders via the inactive cylinders, providing cooled EGR benefits.

TECHNICAL FIELD

This application relates to varying a valve timing to achieve virtualcylinder deactivation and selectively enable reverse flow through adeactivated cylinder bank to achieve EGR benefits.

BACKGROUND AND SUMMARY

Engines may be operated with variable valve timing control to improveengine performance. For example, intake and/or exhaust valve timings maybe adjusted (e.g., advanced or retarded) based on engine operatingconditions to increase positive valve overlap. The increased valveoverlap may then be used for improving air-fuel mixing, cylinder chargetemperature control, etc. During still other conditions, the valvetimings may be adjusted to increase negative valve overlap.

One example approach for controlling an engine variable valve timingdevice is shown by Winstead in U.S. Pat. No. 7,779,823. Therein, a valvetiming of a first engine cylinder is adjusted to flow gases from theengine intake to the engine exhaust while a valve timing of a secondcylinder on the same bank is adjusted to return combusted exhaust gasesback to the intake. In this way, a timing of an auto-ignition combustionin the engine (e.g., when operating in an HCCI mode) can be controlled.

However, the inventors herein have identified potential issues with suchan approach. As one example, cooled EGR may not be achieved.Specifically, since exhaust gases are returned via one or more cylindersof the same bank, the returned exhaust gas may be at a substantiallyhigh temperature. While this may help expedite cylinder heating whilethe engine is in a HCCI combustion mode, during a spark ignitioncombustion mode, the higher temperature recycled exhaust gases can leadto misfires and other abnormal combustion events in the combustingcylinders. As such, this may degrade engine performance.

Thus in one example, some of the above issues may be addressed by amethod of operating an engine comprising operating a first group ofcylinders on a first engine bank to provide a net flow of air andexhaust gas from a first intake manifold to a first exhaust manifoldwhile operating a second group of cylinders on a second engine bank toprovide a net flow of exhaust gas from a second exhaust manifold to asecond intake manifold. In this way, fuel may be combusted on a firstactivated bank of cylinders while exhaust gas is recirculated via asecond deactivated bank of cylinders.

For example, an engine may include a first group of cylinders coupled toa first exhaust catalyst on a first engine bank and a second group ofcylinders coupled to a second exhaust catalyst on a second engine bank.During selected conditions, such as when an engine load is lower than athreshold, fuel may be injected to, and combusted in, the first group ofcylinders. In addition, a valve timing of the first group of cylindersmay be adjusted so as to flow air and exhaust gas from an intakemanifold towards an exhaust junction through the first exhaust catalyst.At the same time, no fuel may be injected to the second group ofcylinders. Instead, a valve timing of the second group of cylinders maybe adjusted so as to recirculate at least some exhaust gas from theexhaust junction to the intake manifold via the second group ofcylinders. That is, flow through the second bank may be in a directionopposite from the flow through the first bank. As such, the exhaust gasmay be cooled as it passages through the cylinders of the deactivatedbank, thereby providing cooled EGR. Optionally, an exhaust air-to-fuelratio at a position between the second exhaust catalyst and the exhaustjunction may be monitored to identify the presence of exhaust leaks.

Additionally, temporary enrichment of the exhaust generated at the firstgroup of cylinders may be advantageously used to at least partiallyregenerate the second catalyst coupled to the second group of cylinders.This reduces the fuel required to regenerate the catalyst uponsubsequent cylinder reactivation.

In this way, combusted exhaust gas generated on a first engine bank maybe recirculated via a second, different engine bank. By reversing flowthrough cylinders of a deactivated bank, the recirculated exhaust gasmay be cooled. Overall, cylinder deactivation and cooled EGR benefitscan be simultaneously provided to improve engine performance. By usingthe reverse flow to detect exhaust leaks, exhaust degradation may alsobe diagnosed concomitantly.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-2 schematically show aspects of an example engine system inaccordance with an embodiment of this disclosure.

FIG. 3 depicts a partial engine view.

FIGS. 4-6 illustrate example methods for adjusting intake and/or exhaustvalve timing for each of a first group and second group of cylinders toreduce flow, or reverse flow, through a deactivated engine bank.

FIG. 7 illustrates an example method for diagnosing an exhaust gas leakin a deactivated engine bank during reverse flow conditions.

FIG. 8 illustrates example valve timing adjustments to each of a firstand second group of cylinders, according to the present disclosure.

FIG. 9 illustrates an example valve timing adjustment to the secondgroup of cylinders to provide substantially zero flow through the secondbank.

DETAILED DESCRIPTION

The following description relates to systems and methods for adjustingintake and/or exhaust valve timing for a first group of cylinders on afirst engine bank and a second group of cylinders on a second enginebank (FIGS. 1-3) to enable selective cylinder deactivation. A controllermay be configured to perform control routines, such as those shown atFIGS. 4-6, to direct substantially less flow, in the same direction,through an inactive bank as compared to an active bank during someoperating conditions. During other conditions, the controller may directcombusted exhaust gas from the active bank through the inactive bank, inthe opposite direction. As shown at FIG. 7, the controller may alsodetect exhaust leaks in the inactive bank during the reversed flow basedon changes in monitored air-to-fuel ratio. By adjusting the valve timingof the inactive bank based on an exhaust air-to-fuel ratio of theinactive bank, cylinder deactivation benefits may be achieved whilemaintaining conditions for exhaust catalyst function. Exampleadjustments are described herein with reference to FIGS. 8-9.

FIG. 1 shows an example engine 10 including a plurality of combustionchambers or cylinders 30. The plurality of cylinders 30 of engine 10 arearranged as groups of cylinders on distinct engine banks. In thedepicted example, engine 10 includes two engine banks 14A, 14B. Thus,the cylinders are arranged as a first group of cylinders arranged onfirst engine bank 14A and a second group of cylinders arranged on secondengine bank 14B.

Engine 10 can receive intake air via an intake passage 42 communicatingwith branched intake manifold 44A, 44B. Specifically, first engine bank14A receives intake air from intake passage 42 via first intake manifold44A while second engine bank 14B receives intake air from intake passage42 via second intake manifold 44B. While engine banks 14A, 14B are shownwith distinct intake manifolds, it will be appreciated that in alternateembodiments, they may share a common intake manifold or a portion of acommon intake manifold. The amount of air supplied to the cylinders ofthe engine can be controlled by adjusting a position of throttle 64.Additionally, as elaborated herein, an amount of air supplied to eachgroup of cylinders on the specific banks can be adjusted by varying anintake valve timing of one or more intake valves coupled to thecylinders, which are shown in greater detail in FIG. 3.

Combustion products generated at the cylinders of first engine bank 14Aare exhausted to the atmosphere via first exhaust manifold 48A. A firstemission control device, such as first exhaust catalyst 70A is coupledto first exhaust manifold 48A. Exhaust gas is directed from first enginebank 14A through first exhaust catalyst 70A towards exhaust junction 55along first exhaust manifold 48A. From there, the exhaust can bedirected to the atmosphere via common exhaust passage 50. Likewise,combustion products generated at the cylinders of second engine bank 14Bare exhausted to the atmosphere via second exhaust manifold 48B. Asecond emission control device, such as second exhaust catalyst 70B iscoupled to second exhaust manifold 48B. Exhaust gas is directed fromsecond engine bank 14B through second exhaust catalyst 70B towardsexhaust junction 55 along second exhaust manifold 48B. From there, theexhaust can be directed to the atmosphere via common exhaust passage 50.

As elaborated below, by adjusting cam timings, cylinder valve timingscan be adjusted to provide virtual cylinder deactivation wherein flowthrough the cylinder is reduced. For example, substantially zero flowthrough the second bank can be provided. As an example, when the camtiming is such that the intake and exhaust opening center onapproximately the bottom of the piston travel, gases (intake air and/orexhaust gas) can flow in and out of the cylinder with a minimal net flowbetween the intake and the exhaust manifolds. However, during suchconditions, minor variations in cam timing, exhaust pressure and intakepressure can result in at least some net flow between the intake and theexhaust manifolds. If the net flow is from the intake to the exhaustsystem, excess oxygen is introduced into the exhaust catalyst whichreduces the NOx conversion efficiency of the catalyst when the cylindersare reactivated, and which leads to a need for excess fuel to beintroduced for catalyst regeneration. This reduces overall VDE gains. Onthe converse, reversed flow from the active bank to the inactive bankmay be used during conditions where cooled EGR is desired. During othertimes, the reverse flow can compromise power output and fuel efficiency.Thus, a desired amount and direction of flow through the inactive bankcan be monitored and maintained by adjusting the cam timing, or cylindervalve timing, based on an exhaust air-to-fuel ratio.

For example, during selected engine conditions, such as during lowengine loads, one or more cylinders of a selected engine bank may beselectively deactivated. This may include deactivating fuel and spark onthe selected engine bank. In addition, an intake and/or exhaust valvetiming may be adjusted so as to provide substantially less flow throughthe inactive engine bank as compared to the active engine bank. Adirection of the substantially less flow may be constantly adjusted,e.g., alternated, such that substantially zero flow is enabled throughthe inactive bank. For example, an intake and/or exhaust valve timing ofthe inactive engine bank may be continually adjusted based on an exhaustair-to-fuel ratio of the inactive engine bank to provide substantiallyno net flow through the inactive bank while an intake and/or exhaustvalve timing of the active engine bank is adjusted to provide a zeroflow (or no net flow) of air and exhaust gas through the active bank.Operating the second group of non-combusting cylinders on the secondbank with valve timing adjusted to provide substantially no flow ofcharge may include, in response to the exhaust air-to-fuel ratio sensedat the second bank being leaner than stoichiometry, adjusting the valvetiming to a first timing to reduce flow of charge from the second intakemanifold to the second exhaust manifold, and in response to the exhaustair-to-fuel ratio sensed at the second bank being at stoichiometry,adjusting the valve timing to a second timing to reduce flow of chargefrom the second exhaust manifold to the second intake manifold. Herein,by providing substantially less flow through the inactive bank, cylinderdeactivation benefits may be provided without degrading efficiency ofthe exhaust catalyst on the inactive bank (e.g., via retention of oxygenon the exhaust catalyst), thereby reducing the need for activeregeneration of the exhaust catalyst during subsequent cylinderreactivation. This reduces the resultant fuel penalty and improvesoverall engine fuel economy.

As elaborated at FIGS. 4-5, the valve timing of the inactive bank may beadjusted based on an exhaust air-to-fuel ratio of the inactive bank toprovide reduced flow to maintain an air-to-fuel ratio of the inactivebank slightly lean. For example, the valve timing of inactive secondengine bank 14B may be adjusted based on the output of second exhaustair-to-fuel ratio sensor 82 to provide substantially no flow bymaintaining the exhaust air-to-fuel ratio of the bank slightly leanerthan stoichiometry. Alternatively, the valve timing may be adjusted tomaintain the exhaust air-to-fuel ratio of inactive engine bank 14Bslightly leaner than an exhaust air-to-fuel ratio of active engine bank14A (e.g., leaner by less than 10%). The exhaust air-to-fuel ratio offirst engine bank 14A may be estimated by first exhaust air-to-fuelratio sensor 72. As such, sensors 72, 82 may be oxygen sensors (such asEGO, HEGO, or UEG sensors) or other appropriate air-to-fuel ratiosensors. In one example, operating the second group of non-combustingcylinders on the second bank with valve timing adjusted to providesubstantially no flow of charge includes, in response to the exhaustair-to-fuel ratio sensed at the second bank being leaner thanstoichiometry, adjusting the valve timing to a first timing to reduceflow of charge from the second intake manifold to the second exhaustmanifold, and in response to the exhaust air-to-fuel ratio sensed at thesecond bank being at stoichiometry, adjusting the valve timing to asecond timing to reduce flow of charge from the second exhaust manifoldto the second intake manifold.

During still other conditions, such as during low engine loads whenexhaust gas recirculation is requested, one or more cylinders of aselected engine bank may be selectively deactivated and additionally, anintake and/or exhaust valve timing of the inactive bank may be adjustedto provide a net flow through the inactive bank in a direction oppositeto the net flow through the active bank. For example, as shown in FIG.2, second engine bank 14B may be deactivated by deactivating fuel andspark to the selected engine bank. Then, an intake and/or exhaust valvetiming of inactive engine bank 14B may be adjusted so that at least aportion of combusted exhaust gas generated at active engine bank 14A isdrawn from the first exhaust manifold 48A into the second exhaustmanifold 48B upstream of exhaust junction 55. Further, the combustedexhaust gas is drawn from the second exhaust manifold 48B into secondintake manifold 44B, via second catalyst 70B. Thus, the first group ofcylinders of first engine bank 14A are operated to provide a net flow ofair and exhaust gas from first intake manifold 44A to first exhaustmanifold 48A while the second group of cylinders of second engine bank14B are operated to provide a net flow of exhaust gas from secondexhaust manifold 48B to second intake manifold 44B. As the exhaust gastravels through the cylinders of the inactive bank, exhaust gas coolingmay occur such that recirculated exhaust gas received via the inactivebank is cooler than exhaust gas received via a dedicated EGR passage.Herein, by drawing a reverse flow through the inactive bank, cooled EGRbenefits may be provided in addition to the cylinder deactivationbenefits. It will be appreciated that in addition to the EGR receivedvia the reverse flow through the inactive bank, additional EGR may beprovided to the active engine bank through an EGR passage coupledbetween the exhaust manifold and the intake manifold (as shown in FIG.3). For example, a common EGR passage (not shown in FIGS. 1-2) may becoupled from downstream of exhaust junction 55 to upstream of intakemanifolds 44A,44B (and downstream of intake throttle 64). However, insome embodiments, each engine bank may have a dedicated EGR passagecoupled between the corresponding intake manifold, downstream of thethrottle, and the corresponding exhaust manifold, upstream of exhaustjunction 55.

As elaborated at FIG. 6, the valve timing of the inactive bank may beadjusted during the reverse flow such that an exhaust air-to-fuel ratioof the active bank is sensed in the intake manifold of the inactivebank. For example, while reversing flow through inactive second enginebank 14B, active engine bank 14A may be operated richer thanstoichiometry for a duration. The valve timing of second engine bank 14Bmay then be adjusted so that the richer than stoichiometry air-to-fuelratio is sensed at second intake air-to-fuel ratio sensor 84 in secondintake manifold 44B. First engine bank 14A may have a similar firstintake air-to-fuel ratio sensor 74 in first intake manifold 44A. Assuch, sensors 74, 84 may be oxygen sensors (such as EGO, HEGO, or UEGsensors) or other appropriate air-to-fuel ratio sensors. By sensing therich exhaust air-to-fuel ratio of the active bank in the intake of theinactive bank, reverse flow can be confirmed. Additionally, duringselected conditions, the first active bank 14A may be temporarilyoperated richer than stoichiometry for a duration to at least partiallyregenerate the exhaust catalyst on the second inactive engine bank 14B.Herein, the duration of enrichment and/or degree of enrichment may bebased on the regeneration state, or oxygen loading state, of the exhaustcatalyst coupled to the inactive bank. For example, the duration may beincreased and the richness may be increased as the oxygen loading stateof the catalyst increases.

As used herein, adjusting the valve timing of intake and/or and exhaustvalve may include adjusting a cam timing where the valves arecam-actuated valves. For example, a camshaft position of a camshaftcoupled to the intake and/or exhaust valves of the first bank may beadjusted to a first position to provide a first cam timing and acorresponding first valve timing that provides a net flow in a firstdirection through the first bank (specifically, from the intake manifoldto the exhaust manifold). At the same time, a camshaft position of acamshaft coupled to the intake and/or exhaust valves of the second bankmay be adjusted to a second, different position to provide a second,different cam timing and a corresponding second, different valve timingthat provides a net flow in a second, opposite direction through thesecond bank (specifically, from the exhaust manifold to the intakemanifold). Alternatively, the second cam timing and the correspondingsecond valve timing may be constantly adjusted between a timing thatprovides a small net flow in the first direction through the second bank(specifically, from the intake manifold to the exhaust manifold) whileproviding a leaner than stoichiometric exhaust air-to-fuel ratio at thesecond bank, and a timing that provides a small net flow in the seconddirection through the second bank (specifically, from the exhaustmanifold to the intake manifold) while providing a stoichiometricexhaust air-to-fuel ratio at the second bank. The constant alternatingbetween the positions allows a substantially zero net flow to beprovided at the second bank while the exhaust air-to-fuel ratio hoversat slightly leaner than stoichiometry.

In this way, based on engine operating conditions, a selected enginebank may be deactivated while a valve timing of the inactive may beadjusted to adjust a flow of air and exhaust gas through the cylindersof the inactive bank. By allowing the flow to be reversed through theinactive bank during some conditions, cooler EGR may be provided whilemaintaining a performance level of an exhaust catalyst of the inactivebank. By allowing flow through the inactive bank to be substantiallyreduced during other conditions, cylinder deactivation may be providedwhile also maintaining the performance level of the exhaust catalyst ofthe inactive bank and reducing regeneration requirements. In this way,tailpipe exhaust emissions and fuel economy is improved.

It will be appreciated that in some embodiments, the exhaust manifoldsmay further include a shut-off valve (not shown) coupled upstream of therespective exhaust catalyst so as to reduce flow through the catalyst.For example, during conditions when a first group of cylinders on thefirst engine bank 14A are deactivated, a first shut-off valve coupledupstream of first exhaust catalyst 70A may be closed to reduce flowthere-through. Likewise, during conditions when a second group ofcylinders on the second engine bank 14B are deactivated, a secondshut-off valve coupled upstream of second exhaust catalyst 70B may beclosed to reduce flow there-through. By reducing flow, oxygen saturationof the catalyst coupled to the inactive engine bank can be decreased.

As such, when the shut-off valve is closed, pressure and vacuum may tendto build in the corresponding exhaust manifold. This increase in exhaustmanifold pressure would increase the pumping work and reduce the fueleconomy benefits achieved via the cylinder deactivation. Thus in someembodiments, a pressure sensor may also be coupled to the exhaustmanifold to detect the pressure changes, and the valve timing of thecylinders on the inactive engine bank may be further fine-tuned to holdthe exhaust manifold pressure at or around a desired pressure.Alternatively, an oxygen sensor, such as an exhaust UEGO sensor, may beused to infer the pressure of the exhaust manifold since the outputvoltage of the UEGO sensor is sensitive to air pressure. Accordingly,the valve timing of the inactive engine bank may be adjusted based onthe output of the oxygen sensor to maintain the exhaust manifoldpressure at the desired value (e.g., at or below a threshold pressure).

FIG. 3 depicts an example embodiment of a combustion chamber or cylinderof internal combustion engine 10 (such as engine 10 of FIGS. 1-2).Engine 10 may receive control parameters from a control system includingcontroller 12 and input from a vehicle operator 130 via an input device132. In this example, input device 132 includes an accelerator pedal anda pedal position sensor 134 for generating a proportional pedal positionsignal PP. Cylinder (herein also “combustion chamber’) 14 of engine 10may include combustion chamber walls 136 with piston 138 positionedtherein. Piston 138 may be coupled to crankshaft 140 so thatreciprocating motion of the piston is translated into rotational motionof the crankshaft. Crankshaft 140 may be coupled to at least one drivewheel of the passenger vehicle via a transmission system. Further, astarter motor may be coupled to crankshaft 140 via a flywheel to enablea starting operation of engine 10.

Cylinder 14 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 may communicate with othercylinders of engine 10 in addition to cylinder 14. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 3 shows engine 10configured with a turbocharger including a compressor 174 arrangedbetween intake passages 142 and 144, and an exhaust turbine 176 arrangedalong exhaust passage 148. Compressor 174 may be at least partiallypowered by exhaust turbine 176 via a shaft 180 where the boosting deviceis configured as a turbocharger. However, in other examples, such aswhere engine 10 is provided with a supercharger, exhaust turbine 176 maybe optionally omitted, where compressor 174 may be powered by mechanicalinput from a motor or the engine. A throttle 20 including a throttleplate 164 may be provided along an intake passage of the engine forvarying the flow rate and/or pressure of intake air provided to theengine cylinders. For example, throttle 20 may be disposed downstream ofcompressor 174 as shown in FIG. 3, or alternatively may be providedupstream of compressor 174.

Exhaust passage 148 may receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 14. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 178although in some embodiments, exhaust gas sensor 128 may be positioneddownstream of emission control device 178. Sensor 128 may be selectedfrom among various suitable sensors for providing an indication ofexhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO(universal or wide-range exhaust gas oxygen), a two-state oxygen sensoror EGO (as depicted), a HEGO (heated EGO), a NOx, HC, or CO sensor, forexample. Emission control device 178 may be a three way catalyst (TWC),NOx trap, various other emission control devices, or combinationsthereof.

Exhaust temperature may be measured by one or more temperature sensors(not shown) located in exhaust passage 148. Alternatively, exhausttemperature may be inferred based on engine operating conditions such asspeed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhausttemperature may be computed by one or more exhaust gas sensors 128. Itmay be appreciated that the exhaust gas temperature may alternatively beestimated by any combination of temperature estimation methods listedherein.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 14 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 14. In some embodiments, eachcylinder of engine 10, including cylinder 14, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 by cam actuation viacam actuation system 151. Similarly, exhaust valve 156 may be controlledby controller 12 via cam actuation system 153. Cam actuation systems 151and 153 may each include one or more cams and may utilize one or more ofcam profile switching (CPS), variable cam timing (VCT), variable valvetiming (VVT) and/or variable valve lift (VVL) systems that may beoperated by controller 12 to vary valve operation. The operation ofintake valve 150 and exhaust valve 156 may be determined by valveposition sensors (not shown) and/or camshaft position sensors 155 and157, respectively. In alternative embodiments, the intake and/or exhaustvalve may be controlled by electric valve actuation. For example,cylinder 14 may alternatively include an intake valve controlled viaelectric valve actuation and an exhaust valve controlled via camactuation including CPS and/or VCT systems. In still other embodiments,the intake and exhaust valves may be controlled by a common valveactuator or actuation system, or a variable valve timing actuator oractuation system. For example, in the embodiments of FIGS. 1-2, theintake valves of cylinders on a first bank may be controlled by a commonvalve actuator while the exhaust valves on the first bank are controlledby a different, common valve actuator. Likewise, the intake valves andexhaust valves of a second bank may have respective common valveactuators.

Cylinder 14 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen, for example, when higher octane fuels orfuels with higher latent enthalpy of vaporization are used. Thecompression ratio may also be increased if direct injection is used dueto its effect on engine knock.

In some embodiments, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 14 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 192 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel as may be the case with some diesel engines.

In some embodiments, each cylinder of engine 10 may be configured withone or more fuel injectors for delivering fuel. As a non-limitingexample, cylinder 14 is shown including one fuel injector 166. Fuelinjector 166 is shown coupled directly to cylinder 14 for injecting fueldirectly therein in proportion to the pulse width of signal FPW receivedfrom controller 12 via electronic driver 168. In this manner, fuelinjector 166 provides what is known as direct injection (hereafter alsoreferred to as “DI”) of fuel into combustion cylinder 14. While FIG. 3shows injector 166 as a side injector, it may also be located overheadof the piston, such as near the position of spark plug 192. Such aposition may improve mixing and combustion when operating the enginewith an alcohol-based fuel due to the lower volatility of somealcohol-based fuels. Alternatively, the injector may be located overheadand near the intake valve to improve mixing. Fuel may be delivered tofuel injector 166 from a high pressure fuel system 8 including fueltanks, fuel pumps, and a fuel rail. Alternatively, fuel may be deliveredby a single stage fuel pump at lower pressure, in which case the timingof the direct fuel injection may be more limited during the compressionstroke than if a high pressure fuel system is used. Further, while notshown, the fuel tanks may have a pressure transducer providing a signalto controller 12.

It will be appreciated that, in an alternate embodiment, injector 166may be a port injector providing fuel into the intake port upstream ofcylinder 14. Further, while the example embodiment shows fuel injectedto the cylinder via a single injector, the engine may alternatively beoperated by injecting fuel via multiple injectors, such as one directinjector and one port injector. In such a configuration, the controllermay vary a relative amount of injection from each injector.

Fuel may be delivered by the injector to the cylinder during a singlecycle of the cylinder. Further, the distribution and/or relative amountof fuel or knock control fluid delivered from the injector may vary withoperating conditions, such as aircharge temperature, as described hereinbelow. Furthermore, for a single combustion event, multiple injectionsof the delivered fuel may be performed per cycle. The multipleinjections may be performed during the compression stroke, intakestroke, or any appropriate combination thereof.

As described above, FIG. 3 shows only one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc.

Fuel tanks in fuel system 8 may hold fuel with different qualities andcompositions. These differences may include different alcohol content,different water content, different octane, different heat ofvaporizations, different fuel blends, and/or combinations thereof etc.In one example, the engine may use gasoline as a first substance and analcohol containing fuel blend such as E85 (which is approximately 85%ethanol and 15% gasoline) or M85 (which is approximately 85% methanoland 15% gasoline) as a second substance. Other alcohol containing fuelscould be a mixture of alcohol and water, a mixture of alcohol, water andgasoline etc. In still another example, both fuels may be alcohol blendswherein the first fuel may be a gasoline alcohol blend with a lowerratio of alcohol than a gasoline alcohol blend of a second fuel with agreater ratio of alcohol, such as E10 (which is approximately 10%ethanol) as a first fuel and E85 (which is approximately 85% ethanol) asa second fuel. Moreover, fuel characteristics of the fuel or knockcontrol fluid stored in the fuel tank may vary frequently. In oneexample, a driver may refill the fuel tank with E85 one day, and E10 thenext, and E50 the next. The day to day variations in tank refilling canthus result in frequently varying fuel compositions, thereby affectingthe fuel composition delivered by injector 166.

The engine may further include one or more exhaust gas recirculationpassages for recirculating a portion of exhaust gas from the engineexhaust to the engine intake. As such, by recirculating some exhaustgas, an engine dilution may be affected which may improve engineperformance by reducing engine knock, peak cylinder combustiontemperatures and pressures, throttling losses, and NOx emissions. In thedepicted embodiment, exhaust gas may be recirculated from exhaustpassage 148 to intake passage 144 via EGR passage 141. The amount of EGRprovided to intake passage 148 may be varied by controller 12 via EGRvalve 143. Further, an EGR sensor 145 may be arranged within the EGRpassage and may provide an indication of one or more pressure,temperature, and concentration of the exhaust gas.

It will be appreciated that while the embodiment of FIG. 3 shows lowpressure (LP-EGR) being provided via an LP-EGR passage coupled betweenthe engine intake upstream of the turbocharger compressor and the engineexhaust downstream of the turbine, in alternate embodiments, the enginemay be configured to also provide high pressure EGR (HP-EGR) via anHP-EGR passage coupled between the engine intake downstream of thecompressor and the engine exhaust upstream of the turbine. In oneexample, an HP-EGR flow may be provided under conditions such as theabsence of boost provided by the turbocharger, while an LP-EGR flow maybe provided during conditions such as in the presence of turbochargerboost and/or when an exhaust gas temperature is above a threshold. Whendistinct HP-EGR and LP-EGR passages are included, the respective EGRflows may be controlled via adjustments to respective EGR valves.

Controller 12 is shown in FIG. 3 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 110 in this particular example, random access memory 112,keep alive memory 114, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 122; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 120 (or other type)coupled to crankshaft 140; throttle position (TP) from a throttleposition sensor; and manifold absolute pressure signal (MAP) from sensor124. Engine speed signal, RPM, may be generated by controller 12 fromsignal PIP. Manifold pressure signal MAP from a manifold pressure sensormay be used to provide an indication of vacuum, or pressure, in theintake manifold. Still other sensors may include fuel level sensors andfuel composition sensors coupled to the fuel tank(s) of the fuel system.

Storage medium read-only memory 110 can be programmed with computerreadable data representing instructions executable by processor 106 forperforming the methods described below as well as other variants thatare anticipated but not specifically listed.

In this way, the system of FIGS. 1-3 enables a method for an enginewherein a first group of cylinders on a first engine bank is operated tocombust and exhaust gases to a catalyst and then to an exhaust junctionwhile operating a second group of cylinders on a second engine bank todraw gases from the exhaust junction, through a second catalyst and thento the intake. The system also enables a method wherein a first group ofcylinders on a first engine bank is operated to combust and exhaustgases to a catalyst and then to an exhaust junction while operating asecond group of cylinders on a second engine bank to flow air through asecond catalyst and then to the exhaust junction, wherein the flow ofair through the second engine bank is smaller than the flow of exhaustgases through the first engine bank.

Now turning to FIG. 4, an example method 400 is shown for adjustingengine operation to enable selective cylinder deactivation, and forfurther adjusting intake and/or exhaust valve timing for variouscylinder groups to either reduce flow through deactivated cylinders orreverse flow through the deactivated cylinders.

At 402, the method includes estimating and/or measuring engine operatingconditions. These may include, for example, engine speed, desired torque(for example, from a pedal-position sensor), manifold pressure (MAP),manifold air flow (MAF), BP, engine temperature, catalyst temperature,intake temperature, spark timing, boost level, air temperature, knocklimits, etc.

At 404, based on the estimated operating conditions, the routine maydetermine an engine mode of operation (e.g., VDE or non-VDE). Inparticular, it may be determined if cylinder deactivation conditionshave been met. As one example, cylinder deactivation conditions may beconfirmed when torque demand is less than a threshold. As such, ifcylinder deactivation conditions are not met at 404, the routine may endwith the engine operating with all cylinders firing.

At 406, upon confirming cylinder deactivation conditions, the routineincludes selecting a group of cylinders and/or an engine bank todeactivate based on the estimated engine operating conditions. Theselection may be based on, for example, which group of cylinders weredeactivated during a previous VDE mode of operation. For example, ifduring the previous cylinder deactivation condition, a first group ofcylinders on a first engine bank were deactivated, then a controller mayselect a second group of cylinders on a second engine bank fordeactivation during the present VDE mode of operation. As anotherexample, the selection may be based on a regeneration state of a firstexhaust catalyst (or emission control device) coupled to the first bankrelative to the regeneration state of a second exhaust catalyst (oremission control device) coupled to the second bank.

Following the selection, at 408, the selected group of cylinders may bedeactivated. Herein, the deactivation may include selectively turningoff fuel injectors of the selected group of cylinders. As elaboratedbelow, the controller may continue to operate (e.g., open or close)intake and exhaust valves of the deactivated cylinders so as to flow airand/or exhaust gases through the deactivated cylinders. In one example,where the engine is a V8 engine, during a VDE mode, the engine may beoperated with one group of cylinders activated (that is, in a V4 mode)while during the non-VDE mode, the engine may be operated with bothgroups of cylinders activated (that is, in a V8 mode).

At 410, the routine includes adjusting intake and/or exhaust valvetiming of the deactivated group of cylinders based on the exhaustair-to-fuel ratio of the deactivated bank so as to substantially reduceflow through the selected bank. Optionally, the controller may alsomaintain a desired exhaust air-to-fuel ratio in the inactive bank. As anexample, the engine may include a first bank with a first group ofcylinders and a second bank with a second group of cylinders, and thecontroller may have selected the second group of cylinders fordeactivation during the VDE mode. Accordingly, the routine includesoperating the first group of cylinders on the first engine bank toprovide a net flow of air and exhaust gas in a first direction whileadjusting a valve timing of the second group of cylinders on the secondengine bank to have substantially less flow through the second bank ascompared to the first bank and maintain a desired air-to-fuel ratio atthe second bank slightly leaner than stoichiometry (or slightly leanerthan the exhaust air-to-fuel ratio of the active bank).

As elaborated at FIG. 5, a direction of flow through the inactive bankmay be constantly adjusted (e.g., alternated) based on an exhaustair-to-fuel ratio sensed at the inactive bank so that substantially zero(or negligible) flow through the second engine bank is provided. Assuch, the substantially less flow through the second engine bank mayinclude a net flow that is a fraction (e.g., less than 10%) of the netflow through the first engine bank and that continually alternatesdirections between the same direction as flow in the first engine bankand in the opposite direction as flow in the first engine bank. Forexample, while the first bank is operated at stoichiometry, a leanerthan stoichiometry exhaust air-to-fuel ratio at the second bank may beuser to infer a small flow of aircharge from the intake manifold to theexhaust manifold. Responsive to the enleanment, the valve timing may beadjusted to reverse flow through the second bank so that a small flow ofcharge goes from the exhaust manifold to the intake manifold, and theair-to-fuel ratio of the second bank returns to stoichiometry. Then,responsive to the stoichiometric exhaust air-to-fuel ratio at the secondbank, a small flow of charge from the exhaust manifold to the intakemanifold may be inferred and the valve timing may be adjusted to reverseflow through the second bank so that a small flow of charge goes fromthe intake manifold to the exhaust manifold, and the air-to-fuel ratioof the second bank is enleaned. In this way, the continuous alternatingof a flow direction causes substantially zero net flow to be provided atthe second engine bank. In addition, the constant adjustment of flowdirection causes the exhaust air-to-fuel ratio at the second engine bankto hover around leaner than stoichiometry.

As such, the net flow through the first engine bank and the secondengine bank may be in the same direction (herein, the first direction)during some conditions, and in the opposite direction during otherconditions. Specifically, the net flow of air and exhaust gas in thefirst direction through the first engine bank may include a net flowfrom a first intake manifold to a first exhaust manifold of the firstbank. The substantially less flow in the second bank may be in the firstdirection, specifically, from a second intake manifold to a secondexhaust manifold of the second bank during some conditions, and thenalternated to be in the second direction, specifically, from the secondexhaust manifold to the second intake manifold of the second bank duringother conditions.

It will be appreciated that in some embodiments, where the exhaustmanifold includes a shut-off valve coupled upstream of the exhaustcatalyst, the controller may also close the shut-off valve to reduceflow of air through the inactive engine bank into the catalyst, therebyreducing oxygen saturation of the exhaust catalyst.

Next at 412, it may be determined if reverse flow conditions have beenmet. Specifically, it may be determined if engine conditions requireflow through the deactivated group of cylinders to be temporarilyreversed. As such, reverse flow through cylinders of a deactivated bankcan be advantageously used during selected engine conditions torecirculate exhaust gas via the cylinders and provide cooled EGRbenefits. This may enable cylinder deactivation and cooled EGR benefitsto be simultaneously provided for added engine performance.

In one example, reverse flow conditions may include an increase in EGRrequested at the active bank. For example, when EGR requested in thefirst group of cylinders is higher than a threshold, reverse flowconditions may be confirmed. As another example, reverse flow conditionsmay be confirmed in response to a request for cooled EGR at the activebank (e.g., cooled EGR being requested in the first group of cylinder).As yet another example, reverse flow conditions may be confirmed afterthe deactivated engine bank has been operated with reduced flow orsubstantially no net flow (as at 410) for a duration.

Additionally, as elaborated at FIG. 8, reverse flow may be requested ifthe oxygen content of an exhaust catalyst coupled to the inactive bankis higher than a threshold so that reverse flow of rich exhaust gas canbe advantageously used to regenerate (e.g., at least partiallyregenerate) the exhaust catalyst. As such, this reduces the fuel penaltyincurred during subsequent cylinder reactivation.

If reverse flow conditions are met, then at 414, and as elaborated atFIG. 6, the routine includes adjusting intake and/or exhaust valvetimings of the deactivated group of cylinders on the selected bank(herein, the second group of cylinders on the second bank) to enablereverse flow through the inactive bank while maintaining a desiredexhaust air-to-fuel ratio. In the present example, where the secondgroup of cylinders on the second engine bank is selected fordeactivation, the controller may operate the first group of cylinders onthe first engine bank to provide a net flow of air and exhaust gas fromthe first intake manifold to the first exhaust manifold while operatingthe second group of cylinders on the second engine bank to provide a netflow of exhaust gas from the second exhaust manifold to the secondintake manifold. As shown at FIGS. 1-2, the first exhaust manifold maybe coupled to the second exhaust manifold at a junction locateddownstream of a first exhaust catalyst of the first bank and a secondexhaust catalyst of the second bank.

In particular, the routine includes operating a first group of cylinderson a first engine bank to combust and exhaust gas to a catalyst and thento an exhaust junction, while operating a second group of cylinders on asecond engine bank to draw gas from the exhaust junction, through asecond catalyst, and then to an intake. As used herein, the intake ofthe second engine bank may be different from the intake of the firstengine bank (as shown at FIGS. 1-2) or the same as the intake of thefirst engine bank.

Next at 416, the routine includes adjusting an amount of external EGRdelivered to the active engine bank based on the reverse flow throughthe second inactive engine bank. As previously elaborated, by drawingexhaust gas from the exhaust junction of the first and second exhaustmanifolds into the intake manifold of the first, inactive bank, exhaustgas may be recirculated via the inactive bank. In addition, therecirculated exhaust gas may be rapidly cooled as it passes through thecylinders of the deactivated engine bank, When the cooled recirculatedexhaust gas is then pumped through the first active engine bank, cooledEGR benefits are provided alongside the cylinder deactivation benefits.This reduces the amount of cooled EGR that has to be delivered to theactive engine bank via a dedicated EGR passage and dedicated EGR cooler,providing addition fuel economy benefits.

In the present example, the first engine bank may include an EGR passagecoupled between the intake and the exhaust, at a point upstream of theexhaust junction, and the controller may adjust an amount of exhaust gasrecirculated to the first engine bank via the EGR passage based on anamount of gas drawn from the exhaust junction through the secondcatalyst of the second engine bank. This allows a total amount of cooledEGR to be maintained. In one example, by providing the reverse flowduring conditions when EGR demand is high, EGR provided via reverse flowthrough the inactive engine bank may be used to supplement EGR providedvia an EGR passage so that the elevated EGR demand can be met.

At 418, the reverse flow can be advantageously used to diagnose forexhaust leaks. As elaborated at FIG. 7, the controller may sense andmonitor an air-to-fuel ratio in the exhaust manifold of the deactivatedengine bank and indicate a leak in the exhaust manifold of thedeactivated engine bank based on the monitored air-to-fuel ratio beingleaner than a threshold level. With reference to the present example,the controller may indicate a leak in the second exhaust manifold (ofthe second inactive engine bank) in response to an air-to-fuel ratiosensed between the second exhaust catalyst and the exhaust junctionbeing leaner than a threshold level.

Returning to 412, if reverse flow conditions are not met, the controllermay continue to operate the engine with substantially reduced flowthrough the deactivated engine bank until cylinder reactivationconditions are met at 420. Cylinder reactivation conditions may beconfirmed in response to, for example, a driver torque demand beinghigher than a threshold level (e.g., during a tip-in). As anotherexample, cylinder reactivation conditions may be confirmed after theengine has been operated with cylinder deactivation (that is, in the VDEmode) for a defined duration. The duration may be based on, for example,an oxygen loading state of the exhaust catalyst of the inactive enginebank (herein the second engine bank).

If cylinder reactivation conditions are met, then at 422, the routineincludes returning fuel injection and spark ignition to the deactivatedengine bank and resuming combustion in the deactivated group ofcylinders.

Now turning to FIG. 5, an example routine 500 is shown for adjustingintake and/or exhaust valve timings on a deactivated engine bank toprovide a net zero flow through the inactive bank relative to the activebank. The routine of FIG. 5 may be performed as part of the routine ofFIG. 4, such as at 410.

At 502, then routine includes operating a first group of cylinders on afirst engine bank to provide a net flow of air and exhaust gas in afirst direction. The first direction includes a net flow from a firstintake manifold to a first exhaust manifold of the first engine bank. Avalve timing (e.g., intake and/or exhaust valve timing) of the firstgroup of cylinders may be adjusted to provide a higher net flow of gases(e.g., air and exhaust gases) in the first direction.

Herein, the first engine bank is an active engine bank and operating thefirst group of cylinders includes injecting fuel to the first group ofcylinders. In particular, fuel injection to the first group of cylindersand valve timing of the first group of cylinders may be adjusted tomaintain an exhaust air-to-fuel ratio in the first engine banksubstantially at stoichiometry. In some embodiments, a controller mayalso adjust a spark timing of the first group of cylinders based on avalve timing of the second group of cylinders to maintain the exhaustair-to-fuel ratio of the first engine bank and maintain a net braketorque.

At 504, the routine includes adjusting a valve timing of a second groupof cylinders on a second engine bank to have substantially no flow inthe second bank as compared to the first bank. Optionally, a desiredair-to-fuel ratio may be maintained at the second bank. Herein, thesecond engine bank is an inactive engine bank and no fuel is injectedinto the second group of cylinders. For example, the second group ofcylinders may have selectively deactivatable fuel injectors which aredeactivated to operate the engine in a VDE mode (while using the firstgroup of cylinders as the active bank).

The substantially zero flow in the second bank may be provided bycontinually adjusting a valve timing of the second bank responsive to anexhaust air-to-fuel ratio of the second bank. As elaborated at FIG. 9,the continuous adjustment allows the exhaust air-to-fuel ratioconstantly be fluctuated between stoichiometry (or the exhaustair-to-fuel ratio of the first engine bank) and leaner thanstoichiometry (or leaner than the exhaust air-to-fuel ratio of the firstengine bank) such that the net flow in the second bank is zero and thenet exhaust air-to-fuel ratio is slightly leaner than stoichiometry.

As used herein, adjusting the valve timing of the second group ofcylinders includes adjusting an intake and/or exhaust valve timing ofthe second group of cylinders. The valve timing is adjusted, aselaborated below at 506-512, to adjust a flow direction and maintain adesired air-to-fuel ratio at the second engine bank. In particular, thevalve timing is adjusted based on an estimated exhaust air-to-fuel ratioof the second bank to maintain the exhaust air-to-fuel ratio of thesecond bank slightly leaner than an air-to-fuel ratio of the first bank.As one example, the valve timing of the first bank may be adjusted tomaintain an exhaust air-to-fuel ratio of the first bank at or aroundstoichiometry while the valve timing of the second bank may be adjustedto maintain an exhaust air-to-fuel ratio of the second bank slightlyleaner than stoichiometry. As another example, the valve timing of thesecond bank may be adjusted to maintain an exhaust air-to-fuel ratio ofthe second bank slightly leaner than an exhaust air-to-fuel ratio of thefirst bank, and in particular, within a range of the exhaust air-to-fuelratio of the first bank (e.g., at less than 5-10% leaner than theexhaust air-to-fuel ratio of the first bank).

At 506, the exhaust air-to-fuel ratio of the inactive second engine bankmay be sensed and it may be determined if the sensed exhaust air-to-fuelratio is leaner than a threshold. For example, it may be determined ifthe sensed exhaust air-to-fuel ratio is leaner than stoichiometry orleaner than the exhaust air-to-fuel ratio of the active first enginebank. If yes, then at 508, the controller may infer that there issubstantially less flow in a first direction through the second enginebank (specifically in a direction from the intake manifold to theexhaust manifold) than flow in the first direction through the firstengine bank. The controller may accordingly adjust the valve timing ofthe second bank to reverse a direction of the substantially less flowfrom the first direction to a second direction (specifically to adirection from the exhaust manifold to the intake manifold of the secondbank).

If the sensed exhaust air-to-fuel ratio of the second bank is not leanerthan the threshold, then at 510 it may be determined if the sensedexhaust air-to-fuel ratio of the second bank is at or around thethreshold. For example, it may be determined if the sensed exhaustair-to-fuel ratio is at or around stoichiometry or at or around theexhaust air-to-fuel ratio of the active first engine bank. If yes, thenat 512, the controller may infer that there is substantially less flowin a second direction through the second engine bank (specifically in adirection from the exhaust manifold to the intake manifold) than flow inthe first direction through the first engine bank. The controller mayaccordingly adjust the valve timing of the second bank to reverse adirection of the substantially less flow from the second direction tothe first direction (specifically to a direction from the intakemanifold to the exhaust manifold of the second bank).

Herein, the existing exhaust gas oxygen sensor is used to control thenet flow through the inactive group of cylinders. In particular, if anet flow in the inactive engine bank from the intake manifold to theexhaust manifold, the exhaust oxygen sensor will react to the fresh aircoming from the intake manifold and indicate a lean air-to-fuel ratio.If the net flow is from the exhaust manifold to the intake manifold, thesensor will continue to detect the air-to-fuel ratio of the exhaustgasses from the other active engine bank (or the air-to-fuel ratio ofthe exhaust gasses from before the cylinder deactivation, based on therate of flow and cylinder deactivation time), which is closer tostoichiometry. If the flow in the vicinity of the oxygen sensor isslightly alternating directions, fresh air from the intake mixed withexhaust gasses from the downstream portions of the exhaust system wouldresult in a slightly lean measure value. Thus, by controlling the camtiming of the inactive engine bank as a function of the sensed exhaustair-to-fuel ratio in the inactive engine bank to constantly alternateand adjust a direction of small flow through the inactive engine bank,flow through the inactive bank can be maintained substantially at zerowhile maintaining the exhaust air-to-fuel ratio of the second bankslightly lean. This reduces the resulting oxygen saturation of theexhaust catalyst on the inactive engine bank, and therefore theregeneration requirement. By reducing the amount of fuel required forregenerating the catalyst, catalytic efficiency and fuel economy isimproved.

In some embodiments, the valve timing of the second group of cylindersmay be further adjusted based on a pressure of the second exhaustmanifold of the second bank. The pressure of the second exhaust manifoldmay be estimated by a pressure sensor coupled to an exhaust catalyst inthe second exhaust manifold. Alternatively, the pressure of the secondexhaust manifold may be estimated by an oxygen sensor coupled to anexhaust catalyst in the second exhaust manifold. The adjusting of thevalve timing based on the exhaust pressure of the second bank mayinclude adjusting the valve timing to maintain the exhaust pressure ofthe second bank lower than a threshold pressure. As such, elevatedexhaust pressures can lead to increased pumping work and consequentlyfuel economy losses. Thus, by maintaining the exhaust pressure of thesecond bank lower than a threshold pressure, pumping work related lossescan be reduced.

In one example, where the engine is configured with cam actuation ofvalves, the intake and/or exhaust valves of the first group of cylindersmay be operated by a first cam and the intake and/or exhaust valves ofthe second group of cylinders may be operated by a second, differentcam. Herein, adjusting an intake and/or exhaust valve timing of thesecond group of cylinders includes adjusting a second cam timing of thesecond cam while maintaining a first cam timing of the first cam. Forexample, the first cam timing of the first group of cylinders may bedetermined based on engine operating conditions (e.g., torque demand) toprovide the desired combustion with an exhaust air-to-fuel ratio that isat or around stoichiometry. Upon setting the first cam to a positioncorresponding to the first cam timing, the first cam position and firstcam timing may be maintained. At the same time, the second cam timingmay be adjusted (e.g., based on the first cam timing and/or the firstexhaust air-to-fuel ratio) to provide substantially zero flow throughthe second engine bank and to maintain the second exhaust air-to-fuelratio slightly leaner the first exhaust air-to-fuel ratio. For example,the first bank of combusting cylinder may be operated with a cam timingthat provides stoichiometric exhaust air-to-fuel ratios. Then, inresponse to the exhaust air-to-fuel ratio sensed at the second bankbeing leaner than stoichiometry, the second cam may be adjusted to asecond cam timing that enables reduced flow of charge from the secondintake manifold to the second exhaust manifold, and in response to theexhaust air-to-fuel ratio sensed at the second bank being atstoichiometry, adjusting the second cam to a third cam timing thatreduces flow of charge from the second exhaust manifold to the secondintake manifold.

As such, the correlation between the first and second exhaustair-to-fuel ratios implies that changes in the exhaust air-to-fuel ratioof the first engine bank may affect the exhaust air-to-fuel ratio of thesecond engine bank as long as there is reduced alternating flow (even ifvery small) through the second engine bank and as long as the valvetimings of the second bank are at the desired settings. For example, ifthere is a sudden and temporary enrichment of the first exhaustair-to-fuel ratio of the first engine bank, there may be a correspondingsudden and temporary enrichment of the second exhaust air-to-fuel ratioof the second engine bank (e.g., during conditions when flow is fromexhaust manifold to intake manifold in second bank). In this case, nofurther valve timing adjustments of the second bank are required as itindicates that the flow through the second bank is being adjusted basedon the flow through the first bank to maintain reduced flow through thesecond bank relative to the first bank.

If, however, there is no correlation between the exhaust air-to-fuelratios, further valve timing adjustments may be required on the secondengine bank. For example, if there is a sudden and temporary enrichmentof the first exhaust air-to-fuel ratio of the first engine bank, but nocorresponding enrichment of the second exhaust air-to-fuel ratio of thesecond engine bank, valve timings of the second bank may need to bereadjusted. The lack of correlation between the exhaust air-to-fuelratios may indicate that the valve timings of the second engine bankhave potentially deviated from the desired settings and flow through thesecond engine is not at the desired reduced flow. Valve timingreadjustments are then performed so that flow through the second bankcan be returned to a reduced flow through the second bank relative tothe first bank.

As one example, an engine controller may operate a first group ofcombusting cylinders on a first bank with valve timing adjusted toprovide a first, higher flow of charge from a first intake manifold to afirst exhaust manifold. At the same time, the controller may operate asecond group of non-combusting cylinders on a second bank with valvetiming adjusted to provide a substantially no flow of charge from asecond intake manifold to a second exhaust manifold. The valve timing ofthe second group of cylinders may be adjusted based on one or more of anexhaust air-to-fuel ratio and an exhaust pressure of the second exhaustmanifold. The controller may also adjusting a spark timing of the firstgroup of cylinders based on a valve timing of the second group ofcylinders to maintain an exhaust air-to-fuel ratio of the first enginebank and maintain a net brake torque. The intake and/or exhaust valvesof the first group of cylinders may be operated via a first camshaft andintake and/or exhaust valves of the second group of cylinders areoperated via a second camshaft. Accordingly, the valve timing of thefirst group of cylinders may be adjusted to a first timing by shiftingthe first camshaft to a first camshaft position, while a valve timing ofthe second group of cylinders is continually adjusted between a second,different timing and a third, different timing by continually shiftingthe second camshaft between a second, different camshaft positionenabling reduced flow in a first direction and a third, differentcamshaft position enabling reduced flow in a second, opposite direction.In this way, the continuous valve adjustments may be used to alternateflow directions of a reduced amount of charge (e.g., air and/or exhaustgas) between an intake and an exhaust manifold of a second group ofnon-combusting cylinders. This allows substantially no net flow ofcharge (that is, negligibly small amount of net flow) to be providedbetween the second intake manifold and the second exhaust manifold.

In an alternate embodiment, the controller may adjust valve timings byusing a cam phaser on only one cam (either the intake or the exhaust)while adjusting the position of a shut-off valve coupled to the exhaustmanifold. For example, the controller may shut off fuel to a bank andchange the phasing of only one cam of the engine bank to create acondition of extreme negative valve overlap that effectively deactivatesthe bank. At the same time, the controller may shut flow out of theexhaust manifold of the inactive engine bank by closing the shut-offvalve. In this way, cylinder deactivation is achieved. By using some camtiming adjustment and some shut-off valve adjustment, the exhaustmanifold can be blown down to a pressure that is close to the intakemanifold pressure, thereby reducing pumping work.

In yet another embodiment, the controller may move both the intake andthe exhaust cams of a selected engine bank to increase negative valveoverlap while the exhaust manifold is closed via the shut-off valve.This would provide for cam phasing cylinder deactivation (or VDE)without requiring cam to be moved in excess of 90 degrees.

As another example, the controller may operate a first group ofcylinders on a first engine bank to combust and exhaust a larger amountof gas to a first catalyst and then to an exhaust junction at a first,higher flow rate; while operating a second group of cylinders on asecond engine bank to not combust and alternate flow direction of asmaller amount of charge between a second catalyst and the exhaustjunction at a second, lower flow rate. Herein, the second, lower flowrate is a fraction of the first, higher flow rate and alternating flowdirection of the smaller amount of charge includes directing the smalleramount of charge at the second, lower flow rate from the exhaustjunction to the second catalyst during a first condition, and directingthe smaller amount of charge at the second, lower flow rate from thesecond catalyst to the exhaust junction during a second condition. Thealternating of the flow direction may be based on an exhaust air-to-fuelratio of the second group of cylinders relative to an exhaustair-to-fuel ratio of the first group of cylinders. For example, thealternating may include adjusting the valve timing of the second groupof cylinders in a first direction when the exhaust air-to-fuel ratio ofthe second group of cylinders is leaner than an exhaust air-to-fuelratio of the first group of cylinders, and adjusting the valve timing ofthe second group of cylinders in a second, opposite direction when theexhaust air-to-fuel ratio of the second group of cylinders is at oraround the exhaust air-to-fuel ratio of the first group of cylinders.

As another example, the adjustment may include adjusting the valvetiming to an initial timing where an exhaust air-to-fuel ratio of thesecond group of cylinders is within a threshold range of the exhaustair-to-fuel ratio of the first group of cylinders, and then readjustingthe valve timing from the initial timing if the exhaust air-to-fuelratio of the second group of cylinders is outside the threshold range ofthe exhaust air-to-fuel ratio of the first group of cylinders to returnthe exhaust air-to-fuel ratio of the first group of cylinders within thethreshold range.

Now turning to FIG. 6, an example routine 600 is shown for adjustingintake and/or exhaust valve timings on a deactivated engine bank toprovide a net reverse flow through the inactive bank relative to theactive bank. The routine of FIG. 6 may be performed as part of theroutine of FIG. 4, such as at 414.

At 602, the routine includes operating a first group of cylinders on afirst engine bank to provide a net flow of air and exhaust gas from afirst intake manifold to a first exhaust manifold. Operating the firstgroup of cylinders includes injecting fuel to the first engine bankwhile adjusting a valve timing of intake and exhaust valves of the firstgroup of cylinders to a first timing to provide a net flow of exhaustgas from the first intake manifold to the first exhaust manifold.

The controller may adjust the valve timing of intake and exhaust valvesof the first group of cylinders to a first timing by adjusting aposition of a first camshaft coupled to the first group of cylinders toa first position. The first timing may be based on estimated engineoperating conditions (e.g., torque demand) as well as a desired exhaustair-to-fuel ratio. In some embodiments, the controller may also adjust aspark timing of the first group of cylinders based on a second valvetiming of the second group of cylinders to maintain the desired exhaustair-to-fuel ratio and maintain a net brake torque. As an example, valvetiming and fuel injection to the first group of cylinders may beadjusted to maintain an exhaust air-to-fuel ratio in the first enginebank substantially at or around stoichiometry.

At 604, the routine includes operating a second group of cylinders on asecond engine bank to provide a net flow of exhaust gas from a secondexhaust manifold to a second intake manifold. In one example, as shownat FIGS. 1-2, the first exhaust manifold may be coupled to the secondexhaust manifold at a junction located downstream of a first exhaustcatalyst of the first bank and a second exhaust catalyst of the secondbank. Operating the second group of cylinders includes not injectingfuel to the second engine bank while adjusting a valve timing of intakeand exhaust valves of the second group of cylinders to a second,different timing to provide a net flow of exhaust gas from the secondexhaust manifold to the second intake manifold. That is, in an oppositedirection to the direction of flow through the first group of cylindersof the first engine bank. The controller may adjust the valve timing ofintake and exhaust valves of the second group of cylinders to a secondtiming by adjusting a position of a second camshaft coupled to thesecond group of cylinders to a second, different position.

As such, the second valve timing of the second group of cylindersenables exhaust to be drawn from the exhaust junction through a secondexhaust catalyst of the second engine bank, into the engine intake. Inother words, exhaust gas recirculation is enabled in the first enginebank via the second engine bank. In addition, since the exhaust gas iscooled during passage through the cylinders of the inactive engine bank,cooled EGR benefits are achieved. As such, during VDE operation, intakemanifold pressure are typically high (that is, there is low vacuum)which can make it difficult to introduce EGR through conventional EGRmethods (such as via an EGR passage). Herein, by using cam phasingadjustments to allow flow through the inactive engine bank to bereversed, cooled EGR can be pumped into the active engine bank even whenthere is minimal to no intake vacuum. That is, the window for EGRbenefits is expanded during operation in the VDE mode.

Since combusted exhaust gas from the first engine bank is drawn into thesecond engine bank, it thus follows that the first exhaust air-to-fuelratio of exhaust gases generated at the first engine bank should bedetectable at the intake manifold of the second engine bank, as long asthere is reverse flow through the second engine bank. In one example, acontroller may confirm that the valve timing of the second group ofcylinders is adjusted to a timing that enables reverse flow by sensingthe first exhaust air-to-fuel ratio of the first group of cylinders atthe exhaust manifold of the first engine bank (e.g., via a first exhaustair-to-fuel ratio sensor coupled to the exhaust manifold of the firstengine bank) as well as at the intake manifold of the second engine bank(e.g., via a second intake air-to-fuel ratio sensor coupled to theintake manifold of the second engine bank).

As such, by virtue of the reverse flow wherein exhaust is delivered tothe exhaust junction from the first engine bank and drawn from theexhaust junction into the second engine bank, changes in the exhaustair-to-fuel ratio of the first engine bank may affect the exhaustair-to-fuel ratio sensed at the second engine bank. In particular, aslong as there is reverse flow through the second engine bank and as longas the valve timings of the second bank are at the desired settingsenabling reverse flow, changes in the exhaust air-to-fuel ratio of thefirst engine bank will correlate with corresponding changes in theexhaust air-to-fuel ratio sensed at the intake of the second enginebank. For example, if there is a sudden and temporary enrichment of thefirst exhaust air-to-fuel ratio of the first engine bank, there may be acorresponding sudden and temporary enrichment of the second exhaustair-to-fuel ratio sensed at the intake of the second engine bank. Inthis case, no further valve timing adjustments of the second bank arerequired as it indicates that the flow through the second bank is beingadjusted based on the flow through the first bank to maintain reverseflow through the second bank relative to the first bank.

If, however, there is no correlation between the exhaust air-to-fuelratios, further valve timing adjustments may be required on the secondengine bank. For example, if there is a sudden and temporary enrichmentof the first exhaust air-to-fuel ratio of the first engine bank, but nocorresponding enrichment of the second intake exhaust air-to-fuel ratiosensed at the intake of the second engine bank, valve timings of thesecond bank may need to be readjusted. The lack of correlation betweenthe exhaust air-to-fuel ratios may indicate that the valve timings ofthe second engine bank have potentially deviated from the desiredsettings and flow through the second engine is not at the desiredreverse flow. Valve timing readjustments are then performed so that flowthrough the second bank can be returned to a reverse flow through thesecond bank relative to the first bank.

In one example, the controller may operate the first group of cylinderson the first engine bank at an air-to-fuel ratio that is richer thanstoichiometry for a duration until the richer than stoichiometryair-to-fuel ratio is sensed at an oxygen sensor in the second intakemanifold of the second group of cylinders. Then, after the duration ofrunning rich, the controller may resume operating the first group ofcylinders at an air-to-fuel ratio that is at or around stoichiometry. Inthis way, reverse flow through the second engine bank is detected andbetter confirmed.

In an alternate embodiment, after the duration of running rich, thecontroller may operate the second group of cylinders on the secondengine bank to provide no net flow of air or exhaust gas between thesecond intake manifold and the second exhaust manifold. For example, thecontroller may shift the second camshaft from the second position to athird position so as to adjust the valve timing of intake and exhaustvalves of the second group of cylinders from the second timing to athird timing, while maintaining the first position of the first camshaftand the first timing of the intake and exhaust valves of the first groupof cylinders.

As another example, a controller may confirm the reverse flow throughthe second engine bank by operating the first group of cylinders richerthan stoichiometry for a duration until an air-to-fuel ratio sensed atthe second intake manifold of the second engine bank is richer than athreshold level. For example, the first group of cylinders may continueto be enriched until the exhaust air-to-fuel ratio sensed at the secondintake manifold of the second engine bank is within a range (e.g.,within 10% of) the exhaust air-to-fuel ratio sensed at the exhaustmanifold of the first engine bank. Herein, the enrichment may be basedon the oxygen loading state of a second exhaust catalyst coupled to thesecond group of cylinders. For example, during a transition into VDEmode, the second catalyst downstream of the inactive second group ofcylinders may be saturated with oxygen within 6 engine revolutions sincethe VCT system takes ˜4-6 cycles to transition from normal flow to noflow (or reduced flow) conditions. While the second catalyst is loadedwith oxygen, the enrichment of the exhaust of the first group ofcylinders may not be sensed in the intake of the second group ofcylinders. Once the fuel from the rich exhaust has displaced the oxygenfrom the second exhaust catalyst, the enrichment may be sensed in thesecond intake and catalyst regeneration may be determined. Then, thefirst group of cylinders may resume operation at stoichiometry. Herein,the rich exhaust gas is advantageously used to reduce the exhaustcatalyst, thereby improving NOx conversion efficiency of the catalystwhen the cylinders are reactivated.

As such, while flow of charge through the second engine bank is beingdirected in a direction that is opposite to the flow of charge throughthe first engine bank, the reverse flow can be advantageously used toidentify exhaust leaks. FIG. 7 shows an example routine 700 that may beused to identify such exhaust leaks based on exhaust air-to-fuel ratiodeviations. As such, the routine of FIG. 7 may be performed as part ofthe routine of FIG. 4, such as at 418. In the present example, a firstgroup of cylinders on a first engine bank are the active cylinders whilea second group of cylinders on a second engine bank are the inactivecylinders.

At 702, the routine includes sensing an air-to-fuel ratio at a locationbetween an exhaust catalyst in the second exhaust manifold and theexhaust junction. That is, an exhaust air-to-fuel ratio sensed by asecond exhaust air-to-fuel ratio sensor in the exhaust manifold of theinactive bank may be monitored. At 704, it may be determined if thesensed air-to-fuel ratio is leaner than a threshold. For example, it maybe determined if the sensed air-to-fuel ratio is leaner than theexpected air-to-fuel ratio. As such, the exhaust air-to-fuel ratioexpected at the second exhaust manifold should be substantially the sameas (e.g., with a range of, such as within 10% of) the exhaustair-to-fuel ratio sensed at the first exhaust manifold of the firstengine bank because during the reverse flow, exhaust is drawn from thefirst exhaust manifold into the second exhaust manifold around theexhaust junction. However, if a leak is present in the exhaust manifold,air maybe unintentionally drawn in and mixed with the exhaust gas,leading to an enleanment of the exhaust air-to-fuel ratio.

If the monitored air-to-fuel ratio is not leaner than the threshold,then at 708, no exhaust leak may be determined. In comparison, at 706,the controller may indicate a post flange exhaust leak in the secondengine bank responsive to the monitored air-to-fuel ratio being leanerthan the threshold. In this way, an unexpected enleanment of exhaustair-to-fuel ratio sensed at the second group of cylinders, during areverse flow through the second bank, can be advantageously used toidentify exhaust leaks.

Now turning to FIG. 8, map 800 shows an example engine operation whereinflow through a deactivated engine bank is adjusted to provide a reducedflow in the same direction as charge flow through an active engine bankduring some conditions, and to provide a reverse flow through theinactive bank in the opposite direction as charge flow through theactive engine bank during other conditions.

Map 800 depicts changes in a first exhaust air-to-fuel ratio (AFR_Bank1)sensed at the first exhaust manifold of a first, combusting group ofcylinders on a first, active engine bank at plot 802. Changes in asecond exhaust air-to-fuel ratio (AFR_Bank2) sensed at the second intakemanifold of a second, non-combusting group of cylinders on a second,inactive engine bank are depicted at plot 804. Plots 802 and 804illustrate enrichment of an air-to-fuel ratio, relative to a baseline(803) representative of stoichiometry as you go above the baseline, andan enleanment as you go below the baseline. Changes to a flow(Flow_Bank2) of gases (air and/or exhaust gases) through the second,non-combusting group of cylinders on the second, inactive engine bankare depicted at plot 806. Valve timing adjustments (VVT_Bank2) to thesecond group of cylinders on the second, inactive engine bank aredepicted at plot 808. Fueling adjustments (Fuel_Bank2) to the secondgroup of cylinders on the second, inactive engine bank are depicted atplot 810. All changes are shown over time (along the x-axis).

Prior to t1, the engine may be operating in a non-VDE mode with allcylinders firing. That is, a first group of cylinders on a first enginebank as well as a second group of cylinders on a second engine bank maybe combusting. According, fueling to both group of cylinders may beadjusted to provide an exhaust air-to-fuel ratio at a first exhaustmanifold of the first engine bank and a second exhaust manifold of thesecond engine bank at or around stoichiometry 803 (the exhaustair-to-fuel ratio of the first group of cylinders is shown at plot 802).Fueling to the second group of cylinders is shown at plot 810 while avalve timing that enables air flow to the second group of cylinders isshown at plots 806 and 808. An air-to-fuel ratio sensed at the intake ofthe second engine bank may be leaner than stoichiometry 803 (plot 804)due a larger amount of intake air available at the intake manifold ascompared to a corresponding exhaust manifold.

At t1, cylinder deactivation conditions may be confirmed and the enginemay shift to operating in a VDE mode with the second group of cylindersselected for deactivation. Accordingly, fuel injection to the secondgroup of cylinders may be deactivated (plot 810). In addition, a valvetiming of the second group of cylinders may be adjusted (plot 808) to atiming to a timing that provides reduced flow of charge through thesecond group of cylinders (plot 806). That is, while fuel is injected tothe first group of cylinders, a valve timing of the first group ofcylinders (not shown) is maintained to provide a higher flow of chargethrough the first engine bank while fuel is not injected to the secondgroup of cylinders, and a valve timing of the second group of cylindersis adjusted to provide a lower flow of charge through the second enginebank. As a result, the controller may operate the first group ofcylinders on the first engine bank to combust and exhaust gas to a firstcatalyst and then to an exhaust junction at a first, higher flow ratewhile operating the second group of cylinders on the second engine bankto not combust and pump air to a second catalyst and then to the exhaustjunction at a second, lower flow rate. The second, lower flow ratethrough the second group of cylinders may include substantially no flow,or may be a flow rate that is a fraction of the first, flow rate throughthe first group of cylinders (e.g., less than 10% of the flow ratethrough the first engine bank).

The controller may adjust the valve timing of the second group ofcylinders based on an exhaust air-to-fuel ratio of the first group ofcylinders to provide reduced flow through the second engine bank whilealso maintaining a desired exhaust air-to-fuel ratio at the second groupof cylinders. For example, the controller may adjust the valve timing toan initial timing where an exhaust air-to-fuel ratio of the second groupof cylinders is within a threshold range of the exhaust air-to-fuelratio of the first group of cylinders (e.g., within +/−10% of theexhaust air-to-fuel ratio of the first group of cylinders). At thisinitial timing, a change in the exhaust air-to-fuel ratio of the firstgroup of cylinders may correlate with a corresponding change in theexhaust air-to-fuel ratio sensed at the second group of cylinders. Forexample, as shown at region 811, a temporary enleanment of the exhaustair-to-fuel ratio of the second group of cylinders occurs (see plot 804within region 811) due to a sudden increase in flow through the secondgroup of cylinders (see plot 806 within region 811), the temporaryenleanment responsive to a corresponding temporary enleanment of theexhaust air-to-fuel ratio of the first group of cylinders (see plot 802within region 811). In other words, as long as both changes areproportional, the flow and exhaust air-to-fuel ratio of the second groupof cylinders is within the threshold range of the flow and exhaustair-to-fuel ratio of the first group of cylinders. Consequently, novalve timing adjustments (see plot 808 within region 811) are requiredto address the temporary enleanment.

In comparison, if there is a change in the exhaust air-to-fuel ratio ofthe second group of cylinders that does not correlate with acorresponding change in the exhaust air-to-fuel ratio of the first groupof cylinders (as a result of which the exhaust air-to-fuel ratio of thefirst group of cylinders falls outside the threshold range), the valvetiming may need to be readjusted. As an example, the controller mayreadjust the valve timing from the initial timing if the exhaustair-to-fuel ratio of the second group of cylinders is outside thethreshold range of the exhaust air-to-fuel ratio of the first group ofcylinders to return the exhaust air-to-fuel ratio of the first group ofcylinders within the threshold range.

One such example adjustment is shown at region 812 wherein a temporaryenleanment of the exhaust air-to-fuel ratio of the second group ofcylinders occurs (see plot 804 within region 812) due to a suddenincrease in flow through the second group of cylinders (see plot 806within region 812), even though there is no corresponding temporaryenleanment of the exhaust air-to-fuel ratio of the first group ofcylinders (see plot 802 within region 812). To address the uncorrelatedtemporary enleanment, the valve timing of the second group of cylindersis adjusted (in a first direction) to reduce flow through the secondgroup of cylinders and return the exhaust air-to-fuel ratio within thethreshold range.

Another example adjustment is shown at region 814 wherein a temporaryenrichment of the exhaust air-to-fuel ratio of the second group ofcylinders occurs (see plot 804 within region 814) due to a suddendecrease in flow through the second group of cylinders (see plot 806within region 814), even though there is no corresponding temporaryenrichment of the exhaust air-to-fuel ratio of the first group ofcylinders (see plot 802 within region 814). To address the uncorrelatedtemporary enrichment, the valve timing of the second group of cylindersis adjusted (in a second direction that is opposite to the firstdirection of the adjustment in the preceding example) to increase flowthrough the second group of cylinders and return the exhaust air-to-fuelratio within the threshold range.

As such, between t1 and t2, as air is pumped through the second group ofcylinders, oxygen loading of a second exhaust catalyst in the exhaustmanifold of the second engine bank may increase. This oxygen loading candecrease the performance of the catalyst and may require regenerationwhen the second group of cylinders is subsequently reactivated.Consequently, a high fuel penalty is incurred. To reduce the fuelpenalty and improve catalyst performance on the inactive bank, at t2,the engine controller may readjust the valve timing of the second groupof cylinders to reverse flow through the second engine bank. In thepresent example, reversal of flow direction is shown by change of plot806 from one side of line 807 (representative of a given direction offlow) to the other side of line 807 (representative of an oppositedirection of flow). That is, the valve timing of the second group ofcylinders is adjusted so that exhaust gas (combusted and generated atthe first group of cylinders) is drawn from the exhaust manifold of thefirst engine bank, through the exhaust junction, and then through thesecond exhaust catalyst into the intake manifold of the second enginebank. That is, exhaust from the first, active engine bank isrecirculated via the second, inactive engine bank.

While reversing flow, the controller may also adjust the injection tothe first group of cylinders to be richer than stoichiometry for aduration. The temporary enrichment of the first exhaust air-to-fuelratio of the first group of cylinders may be based on an amount ofoxygen loaded onto the second exhaust catalyst during the precedingoperation with reduced flow. For example, the controller may estimate anamount of oxygen loaded onto the second exhaust catalyst between t1 andt2 based on the reduced air flow rate through the second group ofcylinders as well as the air-to-fuel ratio of the second group ofcylinders. As the oxygen loading increases, a degree of richness of theenrichment of the first group of cylinders (initiated at t2, as shown inplot 802) may be increased.

Between t2 and t3, the rich exhaust generated at the first group ofcylinders may be drawn, around the exhaust junction (of the first andsecond engine banks), via the exhaust manifold of the second group ofcylinders, into the engine intake. As the rich exhaust passes over andthrough the second exhaust catalyst of the second engine bank, oxygen isdisplaced from the second catalyst and replaced with fuel, therebyregenerating the catalyst. As long as the second catalyst is beingregenerated via the rich exhaust gas, the richening of the first exhaustair-to-fuel ratio does not produce a corresponding richening of theair-to-fuel ratio sensed at the intake of the second group of cylinders(as shown at plots 802 and 804 between t2 to t3). In this way, reverseflow or recirculation of rich exhaust gas from an active engine bank toan inactive engine bank, via an exhaust catalyst of the inactive enginebank, allows the catalyst to be at least partially regenerated. Thisreduces the fuel penalty that would have otherwise been incurred duringsubsequent reactivation of the inactive engine bank.

Once the second exhaust catalyst is regenerated, the richening of thefirst exhaust air-to-fuel ratio produces a corresponding richening ofthe air-to-fuel ratio sensed at the intake of the second group ofcylinders (as shown at plots 802 and 804 from t3 to t4). At t3, upondetecting the rich air-to-fuel ratio of the second engine bank at theintake of the first engine bank, the controller determines that secondcatalyst regeneration has been completed and the enrichment of the firstexhaust air-to-fuel ratio is discontinued. The exhaust air-to-fuel ratioof the first group of cylinders is then returned to being at or aroundstoichiometry (see plot 802 after t3).

At t4, cylinder reactivation conditions may be met. Accordingly, at t4,fuel injection to the second group of cylinders may be resumed (plot810), and valve timing for the second group of cylinders may bereadjusted (plot 808) to allow higher flow of charge through the secondbank (plot 806), in the first direction from the intake to the exhaust.The changes in fueling and air flow to the second engine bank may beadjusted to operate the second group of cylinders at an exhaustair-to-fuel ratio that is substantially at stoichiometry 803 (plot 804).Herein, by regenerating the second catalyst while the second engine bankis deactivated, additional regeneration required upon cylinderreactivation may be reduced. In one example, the catalyst on theinactive engine bank may be partially regenerated during the cylinderdeactivation cycle, the regeneration completed during the subsequentreactivation cycle. By reducing regeneration requirements, fuel economyis improved.

Now turning to FIG. 9, map 900 shows an example engine operation whereinvalve timing adjustments are continually performed based on an exhaustair-to-fuel ratio of a deactivated engine bank to provide substantiallyzero flow through the inactive engine bank.

Map 900 depicts changes in a first exhaust air-to-fuel ratio (AFR_Bank1)sensed at the first exhaust manifold of a first, combusting group ofcylinders on a first, active engine bank at plot 902. Changes in asecond exhaust air-to-fuel ratio (AFR_Bank2) sensed at the secondexhaust manifold of a second, non-combusting group of cylinders on asecond, inactive engine bank are depicted at plot 904. Plots 902 and 904illustrate enrichment of an air-to-fuel ratio, relative to a baseline(903) representative of stoichiometry as you go above the baseline, andan enleanment as you go below the baseline. Valve timing adjustments(VVT_Bank2) to the second group of cylinders on the second, inactiveengine bank are depicted at plot 906. All changes are shown over time(along the x-axis).

In the depicted example, the engine may be operating in a VDE mode withone or more cylinders deactivated. In particular, the engine may beoperating with a first group of cylinders on a first, active engine bankcombusting fuel and with a second group of cylinders on a second,inactive engine bank not combusting fuel. A valve timing of the firstgroup of cylinders (not shown) may be adjusted so that an exhaustair-to-fuel ratio sensed at the first engine bank (plot 902) issubstantially at or around stoichiometry 903. At the same time, a valvetiming of the second group of cylinders (plot 906) may be continuouslyadjusted based on the exhaust air-to-fuel ratio sensed at the secondengine bank (plot 904). In particular, the exhaust air-to-fuel ratiosensed at the second engine bank is used to infer a direction of flowthrough the second engine bank, and accordingly, a valve timingadjustment is made to adjust the direction of flow so that substantiallyzero flow is provided at the second bank.

For example, at each of time points t1, t3, and t5, a leaner thanstoichiometric exhaust air-to-fuel ratio is sensed at the second enginebank. Based on the sensed lean air-to-fuel ratio, the controller mayinfer that there is a net flow of some fresh air from the intakemanifold to the exhaust manifold of the second engine bank. Accordingly,at each of t1, t3, and t5, the controller may adjust the valve timing toreduce and/or reverse flow through the second engine bank. The reversalof flow allows the air-to-fuel ratio of the second bank to be returnedto stoichiometry.

As another example, at each of time points t2 and t4, a stoichiometricexhaust air-to-fuel ratio is sensed at the second engine bank. Based onthe sensed stochiometric air-to-fuel ratio, the controller may inferthat there may be a net flow (or no flow) of some charge from theexhaust manifold to the intake manifold of the second engine bank.Accordingly, at each of t2, and t4, the controller may adjust the valvetiming to reduce and/or reverse flow through the second engine bank. Thereversal of flow allows the air-to-fuel ratio of the second bank to bemoved towards being slightly leaner than stoichiometry. The controllermay then allow the valve timing adjustment to continue until a leanerthan stoichiometry exhaust air-to-fuel ratio is sensed, at which timethe valve timing is adjusted again (but in an opposite direction) toreverse a direction of flow through the second engine bank.

In this way, by continuously adjusting the valve timing, a flowdirection through an inactive engine bank can be alternated toessentially maintain zero net flow through the bank. By reducing forwardflow from the intake manifold to the exhaust manifold of the inactiveengine bank, catalyst oxygen saturation is reduced, thereby reducingregeneration requirements of the catalyst.

In one example, an engine system comprises a first engine bank having afirst group of cylinders, a first intake manifold, a first exhaustmanifold, and a first exhaust catalyst in the first exhaust manifold,and a second engine bank having a second group of cylinders, a secondintake manifold, a second exhaust manifold, and a second exhaustcatalyst in the second exhaust manifold. The second exhaust manifold iscoupled to the first exhaust manifold downstream of a junction, and thesecond intake manifold is coupled to the first intake manifold upstreamof a branch point. The engine system further comprises a first camshaftcoupled to the first engine bank and configured to adjust an intakeand/or exhaust valve timing of the first group of cylinders, as well asa second camshaft coupled to the second engine bank and configured toadjust the intake and/or exhaust valve timing of the second group ofcylinders.

The engine system additionally includes a controller with computerreadable instructions for, injecting fuel to, while adjusting a valvetiming of, the first group of cylinders based on an exhaust air-to-fuelratio of the first exhaust manifold to provide a higher flow of air andexhaust gas from the first intake manifold to the first exhaustmanifold. The controller includes further instructions for not injectingfuel to, while adjusting a valve timing of the second group of cylindersbased on an exhaust air-to-fuel ratio of the second exhaust manifold toprovide substantially lower flow from the second intake manifold to thesecond exhaust manifold. The valve timing of the first group ofcylinders is adjusted to maintain the exhaust air-to-fuel ratio of thefirst exhaust manifold at or around stoichiometry, while the valvetiming of the second group of cylinders is adjusted to maintain theexhaust air-to-fuel ratio of the second exhaust manifold slightly leanerthan stoichiometry.

The controller can also adjust a spark timing of the first group ofcylinders based on the valve timing of the second group of cylinders tomaintain a net brake torque and also to maintain the exhaust air-to-fuelratio of the first exhaust manifold at or around stoichiometry. Afteroperating the second group of cylinders with the substantially zero flowfor a duration, the controller further adjusts the valve timing of thesecond group of cylinders based on an intake air-to-fuel ratio of thesecond intake manifold to draw exhaust gas from the first exhaustmanifold into the second intake manifold via the second exhaustmanifold.

In another example, the controller is configured with instructions forinjecting fuel to, while adjusting a valve timing of, the first group ofcylinders to provide a net flow of air and exhaust gas from the firstintake manifold to the first exhaust manifold. Alongside, the controllermay not injecting fuel to, while adjusting a valve timing of, the secondgroup of cylinders to recirculate exhaust gas from the first exhaustmanifold to the first intake manifold via the second exhaust manifoldand the second intake manifold. In particular, the controller may adjustthe first camshaft to a first position to operate the intake and exhaustvalves of the first group at a first timing, while adjusting the secondcamshaft to a second, different position to operate the intake andexhaust valves of the second group at a second, different timing. Inaddition, while recirculating exhaust gas, the controller may indicatean exhaust leak in the second engine bank responsive to an air-to-fuelratio sensed at the second exhaust catalyst being leaner than athreshold level.

In this way, cam phasing can be used to selectively deactivate a groupof cylinders during a VDE mode of operation. By adjusting a valve timingof an inactive engine bank based on an exhaust air-to-fuel ratio sensedat the inactive bank, flow through an exhaust catalyst can be reducedand substantially zero flow through the inactive bank can be provided.In particular, by continuously adjusting the valve timing based on theexhaust air-to-fuel ratio sensed at the inactive engine bank, pumping offresh intake air from the intake manifold to the exhaust manifold of theinactive bank can be reduced, thereby reducing oxygen saturation of thecatalyst. By adjusting the valve timing during other conditions toreverse flow through the inactive engine bank, cooled EGR benefits canbe provided in addition to VDE benefits, even during low intake vacuumconditions. By enriching the exhaust gas recirculated via the inactiveengine bank, an exhaust catalyst can also be at least partiallyregenerated. By reducing catalyst regeneration requirements duringcylinder reactivation, catalyst efficiency on the inactive bank can beimproved, tailpipe emissions can be reduced, and fuel economy can beimproved.

As will be appreciated by one of ordinary skill in the art, routinesdescribed herein may represent one or more of any number of processingstrategies such as event-driven, interrupt-driven, multi-tasking,multi-threading, and the like. As such, various steps or functionsillustrated may be performed in the sequence illustrated, in parallel,or in some cases omitted. Likewise, the order of processing is notnecessarily required to achieve the objects, features, and advantagesdescribed herein, but is provided for ease of illustration anddescription. Although not explicitly illustrated, one of ordinary skillin the art will recognize that one or more of the illustrated steps orfunctions may be repeatedly performed depending on the particularstrategy being used.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

1. An engine method, comprising: operating a first group of cylinders ona first engine bank to provide a net flow of air and exhaust gas from afirst intake manifold to a first exhaust manifold while operating asecond group of cylinders on a second engine bank to provide a net flowof exhaust gas from a second exhaust manifold to a second intakemanifold.
 2. The method of claim 1, wherein the first exhaust manifoldis coupled to the second exhaust manifold at a junction locateddownstream of a first exhaust catalyst of the first bank and a secondexhaust catalyst of the second bank.
 3. The method of claim 2, whereinoperating the first group of cylinders includes injecting fuel to thefirst engine bank while adjusting a valve timing of intake and exhaustvalves of the first group of cylinders to a first timing to provide anet flow of exhaust gas from the first intake manifold to the firstexhaust manifold, and wherein operating the second group of cylindersincludes not injecting fuel to the second engine bank while adjusting avalve timing of intake and exhaust valves of the second group ofcylinders to a second, different timing to provide a net flow of exhaustgas from the second exhaust manifold to the second intake manifold. 4.The method of claim 3, wherein adjusting the valve timing of intake andexhaust valves of the first group of cylinders to a first timingincludes adjusting a position of a first camshaft coupled to the firstgroup of cylinders to a first position, and wherein adjusting the valvetiming of intake and exhaust valves of the second group of cylinders toa second timing includes adjusting a position of a second camshaftcoupled to the second group of cylinders to a second, differentposition.
 5. The method of claim 4, further comprising, adjusting aspark timing of the first group of cylinders based on the second valvetiming of the second group of cylinders.
 6. The method of claim 5,wherein operating the first group of cylinders on the first engine bankincludes operating the first group of cylinders at an air-to-fuel ratiothat is richer than stoichiometry for a duration until the richer thanstoichiometry air-to-fuel ratio is sensed at an oxygen sensor in thesecond intake manifold of the second group of cylinders.
 7. The methodof claim 6, further comprising, after the duration, operating the firstgroup of cylinders at an air-to-fuel ratio that is at or aroundstoichiometry.
 8. The method of claim 6, further comprising, after theduration, operating the second group of cylinders on the second enginebank to provide no net flow of air or exhaust gas between the secondintake manifold and the second exhaust manifold.
 9. The method of claim8, wherein operating the second group of cylinders to provide no netflow includes shifting the second camshaft from the second position to athird position so as to adjust the valve timing of intake and exhaustvalves of the second group of cylinders from the second timing to athird timing, while maintaining the first position of the first camshaftand the first timing of the intake and exhaust valves of the first groupof cylinders.
 10. The method of claim 2, further comprising, indicatinga leak in the second exhaust manifold in response to an air-to-fuelratio sensed between the second exhaust catalyst and the junction beingleaner than a threshold level.
 11. An engine method, comprising:operating a first group of cylinders on a first engine bank to providecharge flow in a first direction; during a first condition, operating asecond group of cylinders on a second engine bank to provide a chargeflow in a second, opposite direction; and during a second condition,operating the second group of cylinders to provide no net flow throughthe second engine bank.
 12. The method of claim 11, wherein the firstdirection of charge flow includes flowing charge from a first intakemanifold to a first exhaust manifold of the first engine bank, andwherein the second direction of charge flow includes flowing charge froma second exhaust manifold to a second intake manifold of the secondengine bank.
 13. The method of claim 12, wherein the first conditionincludes EGR requested in the first group of cylinders being higher thana threshold level and the second condition includes EGR requested in thefirst group of cylinders being lower than the threshold level.
 14. Themethod of claim 12, wherein the first condition includes cooled EGRrequested in the first group of cylinders and the second conditionincludes no cooled EGR requested in the first group of cylinders. 15.The method of claim 12, wherein the first condition includes the secondgroup of cylinders having been operated with no net flow for a duration.16. The method of claim 12, wherein during the first condition, thefirst group of cylinders are operated richer than stoichiometry for aduration until an air-to-fuel ratio sensed at the second intake manifoldof the second engine bank is richer than a threshold level.
 17. Themethod of claim 12, wherein the second exhaust manifold is coupled thefirst exhaust manifold downstream of a junction, the method furthercomprising, during the first condition, sensing an air-to-fuel ratio ata location between an exhaust catalyst in the second exhaust manifoldand the junction; and indicating a post flange exhaust leak in thesecond engine bank responsive to the monitored air-to-fuel ratio beingleaner than a threshold level.
 18. An engine method, comprising:operating a first group of cylinders on a first engine bank to combustand exhaust gas to a catalyst and then to an exhaust junction; whileoperating a second group of cylinders on a second engine bank to drawgas from the exhaust junction, through a second catalyst, and then to anintake.
 19. The method of claim 18, wherein an intake of the secondengine bank is same as or different from the intake of the first enginebank.
 20. The method of claim 18, wherein the first engine bank includesan EGR passage coupled between the intake and upstream of the exhaustjunction, the method further comprising, adjusting an amount of gasrecirculated on the first engine bank via the EGR passage based on anamount of gas drawn from the exhaust junction through the secondcatalyst of the second engine bank.